The present invention relates to a torque resistant grommet that permits the use of relatively high torque settings in driving fasteners thereinto.
When connecting a part to a sheet metal framework, such as in the installation of automobile parts and accessories, it is common to use an expandable grommet inserted into an aperture formed through a primary panel of the sheet metal framework. A threaded fastener such as a standard screw is passed through the part and driven into the expandable grommet. As the threaded fastener is driven into the grommet, the shank of the grommet expands, thereby preventing the grommet from being extracted from the aperture in the primary panel. Unfortunately, grommets of this type are typically manufactured from relatively soft and inexpensive materials such as polyethylene, nylon, and the like. Consequently, the application of high levels of torque to a threaded fastener being driven into a grommet all too frequently exceeds the level of torque at which the fastener strips out the interior of the grommet, thereby preventing a good hold between the threaded fastener and the grommet.
Accordingly, it is desirable to provide a torque resistant grommet that may be pre-assembled with a primary panel of a structure such as an automobile frame, that is resistant to spinning within an aperture into which it is received so as to allow a threaded fastener to be driven thereinto, and which is furthermore resistant to the stripping of interior of the grommet.
A grommet constructed according to the principles of the present invention has a shank and a head that extends laterally therefrom. The shank is constructed and arranged for-retained insertion in an aperture formed in a structure and the head of the grommet acts to limit the insertion of the shank into the aperture.
The shank itself is made up of an inner sleeve and an outer sleeve. The outer sleeve forms the exterior of the shank and the interior sleeve is disposed within the outer sleeve. The inner sleeve has a bore formed therein that is constructed and arranged to threadedly receive a threaded fastener. In addition, the inner sleeve is secured within the outer sleeve such that a predetermined torque level, referred to as a release torque, will cause the inner sleeve spin within the outer sleeve. This spinning action limits the amount of torque that a threaded fastener may exert on the inner sleeve of the grommet. As the release torque is preferably set below the level of torque at which a threaded fastener will strip out threads formed in the bore of the inner sleeve, the spinning inner sleeve prevents stripping of the grommet.
The shank preferably has a cross-sectional shape that is complementary to the shape of the aperture of the structure into which the grommet is inserted. What is more, the shank of the grommet will be constructed and arranged to resist spinning within the aperture.
Another feature of the present invention is the inclusion of at least one retaining structure on the shank of the grommet. This retaining structure acts to retain the shank of the grommet within the aperture so that the grommet may be pre-assembled with the structure into which the aperture is formed. The retaining structure may take many forms, among which are included a flexible vane, a flexible finger, a ramped projection, an outwardly extending ridge, and a plurality of annular projections.
In order to ensure that the inner sleeve of the grommet shank is not pulled from the outer sleeve of the grommet after the release torque has been exceeded, it is preferred to form a laterally projecting collar that extends from a bottom end of the inner sleeve. The collar is retained within a complementarily shaped channel formed on the interior surface of the outer sleeve. In this way, the inner sleeve may rotate within the outer sleeve without being pulled therefrom.
The release torque may be set in many different manners, including, but not limited to adhering the outer sleeve to the inner sleeve, mechanically or thermoplastically securing the outer sleeve to the inner sleeve, and forming on the exterior of the inner sleeve and/or on the interior of the exterior sleeve at least one projection that contacts the other sleeve in such a manner as to prevent relative rotation therebetween. In a preferred embodiment, the outer surface of the inner sleeve has formed therein a plurality of projections that have a lower release torque in a clockwise direction and a higher release torque in a counter-clockwise direction. Alternatively, the higher and lower torque directions may be reversed.
In order to prevent a threaded fastener from stripping the inner sleeve of the grommet and thereby damaging it, it is preferred to form-the inner sleeve of a relatively hard material. And, as the outer sleeve has more need to deform, the outer sleeve will typically be fashioned of a relatively softer material. The grommet may be formed using an over-molding process or a two-shot injection process. Alternatively, the inner sleeve and outer sleeve are formed independent of one another and subsequently assembled.